This invention is related to co-pending U.S. application Ser. No. 92,024 filed on Nov. 7, 1979 and now U.S. Pat. No. 4,332,164 and assigned to the same assignee.
This invention relates to a gas flow measuring device, or more in particular to a device for measuring the flow rate of an engine intake air.
In conventional devices, an electric heater is provided on the intake manifold of the engine, and temperature-dependent resistors are arranged before and after the heater, thereby to detect the flow rate of the intake air (the gas to be measured).
Such conventional devices have the advantage of a small and simple construction for measuring the flow rate by weight of the intake air. In view of the fact that the electric heater and the temperature dependent resistors are made of resistance wires of the same temperature coefficient, however, the effect of the heat of the intake air cannot be compensated for.
An electric wiring diagram for explaining the principle of a prior art gas flow rate measuring device is shown in FIG. 1. In the construction shown in FIG. 1, reference numeral 10 shows an electric heater having a resistance value of R.sub.H (.OMEGA.), numeral 11 a first temperature dependent resistor having a resistance value of R.sub.1 (.OMEGA.), numeral 12 a second temperature dependent resistor having a resistance value of R.sub.2 (.OMEGA.), and numerals 21 and 22 are reference resistors of the resistance values of R.sub.3 and R.sub.4 (.OMEGA.) respectively making up a bridge circuit together with the first and second temperature dependent resistors 11 and 12. Both the electric heater 10 and the temperature dependent resistors 11 and 12 are formed of a resistor wire having the same temperature coefficient of .alpha.. When the intake air of Ta.degree.C. is heated by the electric heater 10 and the temperature thereof increases by .DELTA.T.degree.C., the temperatures of the first and second temperature dependent resistors 11 and 12 increase to Ta+.DELTA.T and Ta respectively, so that the resistors R.sub.H, R.sub.1 and R.sub.2 are expressed as shown below respectively. EQU R.sub.H =R.sub.OH (1+.alpha..multidot.Ta+.alpha..multidot..DELTA.T.sub.H)(1) EQU R.sub.1 =R.sub.01 (1+.alpha..multidot.Ta+.alpha..multidot..DELTA.T) (2) EQU R.sub.2 =R.sub.02 (1+.alpha..multidot.Ta) (3)
where R.sub.OH, R.sub.01 and R.sub.02 are the values of the resistors R.sub.H, R.sub.1 and R.sub.2 respectively at the temperature of 0.degree. C., and .DELTA.T.sub.H is a temperature increase of the electric heater due to the intake air temperature Ta. Assuming that the electric potentials at the diagonal points a and b of the bridge circuit are V.sub.1 and V.sub.2 respectively, the bridge output voltage .DELTA.V=V.sub.1 -V.sub.2 is given as shown below. ##EQU1## where V is the voltage applied to the bridge and the electric heater.
From equations (2) to (4), if R.sub.3 =R.sub.4 and R.sub.01 =R.sub.02, the temperature difference .DELTA.T is expressed as ##EQU2## If the heat conduction to other than the air from the electric heater is ignored, on the other hand, the relation between the intake air amoung G (g/sec), the temperature difference .DELTA.T and the applied voltage V is given by the equation below. EQU G.multidot.Cp.multidot..DELTA.T=K.sub.1 .multidot.I.sup.2 .multidot.R.sub.H ( 6)
where Cp is a constant pressure specific heat of the air, K.sub.1 a constant and I the current flowing in the electric heater. By erasing .DELTA.T from the equations (5) and (6), the equation below is obtained. ##EQU3## In the operation of this device, .DELTA.V&lt;&lt;V and the value .DELTA.V controlled at a fixed value, and therefore the equation (7) is rewritten as below. ##EQU4## where K.sub.3 is a constant. Alternatively, it is given as below. ##EQU5## where K.sub.3 ' is a constant.
The amount of the intake air is a function of the cube of the current flowing in the electric heater 10 or a function of the cube of the voltage applied to the electric heater 10. The coefficient concerned has a term of the intake air temperature Ta, and for the purpose of accurate measurement of the amount of intake air, the problem is a measuring error resulting from the fact that the term of the intake air temperature is not completely eliminated.